Peter Duffy offers eccentric characters, suspense, and details of spying in what is also cultural history, “Double Agent.”

By Nick Romeo
Globe Correspondent
August 12, 2014

Madison Square Garden might seem an unlikely spot for a massive display of pro-Nazi sentiment. But early in 1939, roughly 20,000 people filled the venue to hear speeches praising Hitler and Mussolini. They cheered as speakers decried the evils of Jewish domination in finance and politics, and at one point they even performed a call-and-response routine featuring the words “sieg” and “heil.”

The demonstration garnered headlines and attention, but a subtle, secretive, and far more dangerous Nazi presence was already operating within US borders. In “Double Agent: The First Hero of World War II and How the FBI Outwitted and Destroyed a Nazi Spy Ring,’’ Peter Duffy tells the intricate tale of FBI counterespionage efforts to disrupt the work of German operatives seeking to steal US military secrets in the years before and during the war.

Before the invention of the atomic bomb, the US military’s most valued piece of technology was the Norden bombsight. Time magazine reported that US aviators claimed the aiming device was so accurate it could allow them to hit a barrel with a bomb from 18,000 feet. In 1939, President Franklin D. Roosevelt said, “We have just one secret, and that is the question of the bombsight.” He was not aware that a Nazi spy had already smuggled technical drawings of the bombsight to aviation experts in the German military.

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The core of the book centers on the theft of the bombsight design by German agents and the FBI’s eventual discovery of the espionage ring that perpetrated it. Duffy traces the trajectories of many key players with various allegiances on both sides of the Atlantic. But his hero is a man named William Sebold, the first double agent in the history of the FBI. The German-American was ostensibly funneling information from a range of US sources back to German intelligence officials. In fact, he and the FBI fooled the Germans into believing they had a loyal operative. Not only was he able to monitor and limit the information German intelligence received, his testimony also proved crucial in convicting many Nazi spies at a later trial.

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Duffy’s story is structured chronologically, beginning with the bombsight’s development in the 1920s by Carl L. Norden Inc. and ending with Sebold’s death in 1970. Norden hired German-Americans and other Northern Europeans, convinced that they alone possessed the necessary skill as craftsmen and engineers. Sometimes, however, they also possessed loyalties to Germany. One worker snuck bombsight plans from the Norden plant, copied them by hand at his kitchen table, and returned them the next morning. The worker’s German spymaster eluded the scrutiny of US customs by curling the plans inside an umbrella cane and sailed back to Germany.

Early in the 1930s, the American government had almost no counterespionage capacities. The OSS (Office of Strategic Services), the predecessor of the postwar Central Intelligence Agency, had not yet been created, and public opinion was generally opposed to granting the government the authority to monitor anyone deemed suspect. By the end of the decade, however, J. Edgar Hoover had persuaded Roosevelt to empower the FBI with jurisdiction over counterespionage and a sizable budget to finance operations. Public opinion was shifting as well, in part because of a well-orchestrated show trial of alleged Nazi spies that the media covered relentlessly.

The book is rich with eccentric characters, suspense, and details of spycraft in the war’s early days. Invisible ink, two-way mirrors, and intricate encryption procedures involving a best-selling novel are among the many tactics Duffy describes. He also provides crisp and engaging accounts of the political and cultural pressures that transformed Germany and the United States before the war. The result is a compelling cultural history with all the intricacy and intrigue of a good spy novel.

The Germans did develop a bombsight inspired by the Norden design; it was operational over the Soviet Union at the very moment that the FBI apprehended many of the spies smuggling information to Germany. But the bureau disrupted and delayed the flow of crucial information to the Nazis, and Sebold’s cool handling of many delicate and combustible situations ultimately justifies Duffy’s claim that he was the first hero of World War II.